A basic setup guide for Plex integratoin is covered in the config guide:
https://rclone.org/drive/#i-class-fa-fa-google-i-google-drive
but this only covers the rclone side of things though, not the Plex setup.
Aside from that I'm sure there exists setup guides if you search around. Perhaps this thread can be useful to you:
@Animosity022 is an experienced user and forum-regular who also uses Plex, so he can probably help if you get stuck.
I don't know enough about Plexdrive to say that is it "obsolete" , but from what I understand rclone is generally considered the better tool for the job and should be able to replace it's function.
The main benefit to encryption is more privacy and security - especially if you don't trust Google that much. Doubly so if you have content on the drive that may be subject to copyright. While Google does not seem to (currently) be very aggressive in this regard, they almost certainly do periodic scans off hashes - and if your drive lights up with an excessive amount of hash-hits for known copyrighted material then they are well within their rights to ban your access next time they bother to do a cleanup. Encryption does not have a lot of downsides really. It uses some CPU cycles to encrypt/decrypt on the fly, but this is fairly trivial on a modern PC and only really an issue on micro-servers like a PI for example. Encryption won't slow down transfers or or make streaming slower. The biggest inconvenience may simply be that you can't really share files directly with others without also sharing your encryption key (which you may not want).
In general it is ok for apps to write directly to the Gdrive. Especially if you use cache-mode writes as that allows full emulation of file-operations, so as far as the apps are concerned it will function just like any other drive, just a bit slower. However, you should take special note of any software that writes files piecemeal or uses temporary work files (like torrents, some rendering software ect.). Rclone will assume a file is done when it is closed and upload it, but if that file is only partially done and will be accessed again very soon then you could end up transferring that file back and fourth dozens of times, and that is horribly inefficient. Whenever possible, use temporary workfolders for unfinished files for such programs. Many torrent programs for example have this built into options and will handle moving finished files to their final destination on their own (I use qbittorrent for this).